Steam turbines have been commonly applied to generation of mechanical or electrical power for over one hundred years. The standard cycle is based upon a source of heat energy to generate steam, a turbine, a water or air cooled condenser for heat rejection, and a pumping system. Steam turbines are highly efficient as the expansive force of steam is the greatest of any of the common gases used for powering turbines. Steam turbines also benefit from use of an inexpensive, plentiful, and environmentally friendly working fluid. Thus, steam turbines are used in many applications.
However, achievement of the highest possible efficiencies requires that high temperatures and high pressures be utilized. In turn, robust operation of steam turbines under these conditions can be problematic. For example, inlet temperatures and pressures of 1400 degrees Fahrenheit (760° C.) and 5600 psi have been used. Common conditions for a modern boiler and steam turbine system are approximately 1050 F. (565 C.) and 2400 psi. This type of system would normally incorporate “reheat” wherein the steam reenters the boiler for one or more stages of heat addition.
Typically, the first turbine section downstream of the boiler and up-stream of the first reheat is referred to as the high pressure (HP) turbine. Exhaust steam from the high pressure (HP) turbine is sent to the boiler for reheating along a cold reheat line. The reheated steam is typically heated to the initial inlet temperature before flowing into an intermediate pressure (IP) turbine. Exhaust from the IP turbine enters and flows through the low pressure (LP) turbine prior to exhaust to the condenser. Some systems may not incorporate the IP section, and more complex systems may have multiple reheat stages. Physical design of the system can vary dependent upon the application. Turbine sections can reside within the same casing, or multiple casings may exist.
A main output shaft, and an area proximate to the spinning steam turbine rotor, typically include bearings designed to handle high temperatures and high pressures. These bearings normally include internal oil seals located between the bearing and the output shaft. In addition, a “thrust” bearing is required to absorb the axial load developed by the power train. This bearing is held in place, or held in a limited range of movement, by axial thrust force and by hydraulic force of the oil in the bearing. This thrust force is created through a combination of the fluid inertia on the turbine buckets and the pressure developed by variation in cross-sectional area activated by using excess steam from the overall system. As the respective bearings may only withstand certain temperatures and pressures of steam, the thrust pressure applied and resultant from the steam, must be within permissible temperature and pressure parameters. Thus, suitable temperature cooling steam from the system may be used to cool areas of the turbine and to provide pressure.
An additional consideration associated with thrust bearings is that thrust bearings do not readily accept multiple and repeated directional changes in thrust due to the existence of a near-zero thrust region wherein the bearing may become metastable. This relationship is shown in FIG. 3. In other words, thrust bearings are designed to be pressurized in a stable manner from one direction or the other. Their ability to rapidly absorb directional reversals in thrust is limited. It is noted herein that substantial damages may occur when steam turbine bearings fail.
Therefore, it is a challenge to ensure that only acceptable pressures and temperatures of steam, including cooling steam, are present in appropriate parts of the system. In response, in the art, turbines and their associated bearings are typically designed and optimized from the outset for a specific set of conditions. For example, a certain size and thrust load capability of a bearing is specified and safety margins are specified by design. However, due to anomalies and also due to standard differences in operating conditions, the reliability of modern steam turbines can still be improved, i.e., start-up verses steady state, failure of the oil seals upon exposure to temperatures beyond design limits, extreme steam temperatures and pressures, vibration, bearing wear, and due to manufacturing variations and other anomalous conditions. It is necessary to ensure that all turbines manufactured achieve their operational and reliability requirements. A single variation from this requirement can be commercially consequential to a steam turbine manufacturer.
Thus in summary, prior art strategies typically attempt to accommodate anomalies and changes in temperature, pressure, and thrust load on a bearing, such as a thrust bearing, by specifying a large or oversized thrust bearing or by compromising on other design goals such as system efficiency or lowest achievable cost. The amount of steam pressure on the bearing or in the various stages of the turbine is typically chosen as a fixed parameter by original design, and is set up for expected conditions including steam cooling requirements. This may be thought of as a passive pressure control strategy and system. Thus, an active pressure and/or thrust control system for a steam turbine is needed.